'We can talk to each other some time' : in UK English some time is two words; yes, you need to use 'to' talk . . 'some day' is all right; 'one day' can be used, but suggests that it will be some time, perhaps rather long, in the future

One-word and two-word forms are not always interchangeable, but here's some of advice - if in doubt, use the two word form. Don't try to learn confusing rules about the supposed differences. If you write 'some time' and 'some day' as two words, you are almost sure to be correct.

Do I need to put 'to' before each other? Yes, or you can say "We can talk sometime". I didn't know the differences between some day/someday and between some time/sometime, so I found some links, hope it helps. But I think they are all more or less interchangeable. http://grammarist.com/usage/someday-some-day/ http://www.grammarly.com/handbook/grammar/adjectives-and-adverbs/19/sometime-sometimes-and-some-time/